Direct conversion receivers DCR are substantially known in the art. They typically comprise a front end unit for receiving the multi-carrier signal from the antenna and a quadrature demodulator for down-converting said received multi-carrier signal to a base band. Disadvantageously the down-converting process results in that the first carrier signal includes an image signal of said second carrier signal and vice versa.
FIG. 7 illustrates the generation of the image signals I1, I2 during a conventional quadrature demodulation process. In the left part of FIG. 7 there is shown the multi-carrier signal as received via the antenna. It comprises for example a first carrier signal C1 and a second carrier signal C2 which are both free of image signals. These carrier signals are located at a first and a second mirror carrier frequency, respectively. These mirror carrier frequencies are arranged symmetrically around a center frequency fc. In the right-hand part of FIG. 7 there are shown the carrier signals after the converting process as carried out by the quadrature demodulator. It can be seen that the down-converted first carrier signal C1 comprises an image signal 12 of the second carrier signal C2 and that said second carrier signal comprises an image signal I1 of the second carrier signal C2. These image signals are undesired because they make a reconstruction of the desired carrier signal more difficult. The difficulty of reconstruction depends on the ratio of image signal to carrier signal which itself depends on the level of impairments in the realisation of the analog quadrature demodulator (gain, phase & quadrature mismatches) as well as similar defects in the A to D converter chain.
The traditional direct conversion receiver DCR further comprises an analog/digital converter for digitizing the down-converted multi-carrier signal and at least a first and a second down-converter DDC-unit for separating the first and the second carrier signal from the digitized down-converted multi-carrier signal and for translating the separated first and second carrier signal to be centered at a predetermined frequency in the frequency plane. In the right-hand part of FIG. 7 said predetermined frequency is set to 0.
In the prior art several methods are known to reject the undesired image signals. For quadrature modulators such a method is known for example from the article “Automatic Adjustment of Quadrature Modulators” by Faulkner, Mattsson and Yates in “Electronic Letters”, vol. 27 no. 3, 31st Jan. 1991. The method of image rejection disclosed in said article can substantially also be applied to quadrature demodulators. However, said method requires quite complex electronic circuits and/or a special signal for calibration. Further, the method of image rejection proposed by said article is made iteratively with the result that it can not be applied to fast-time varying scenarios, such as those encountered for example in multicarrier receivers used according to GSM standard with fast frequency hopping and uplink discontinuous transmission.
Starting from that prior art it is the object of the invention to improve a known direct conversion receiver and a method for receiving a multi-carrier signal, a computer program for carrying out said method as well as an error estimating unit and a compensator stage of such a direct conversion receiver such that image rejection can be carried out less complex and with better accuracy and stability.
This object is solved by the subject-matter of claim 1. More specifically, for the above-described direct conversion receiver this object is solved by providing an error estimating unit for calculating a compensating complex coefficient representing the quota and/or the phase position of the image signal of the second carrier signal being included in the first carrier signal and at least a first compensator stage for removing the undesired image signal from the first carrier signal in response to said compensating coefficient.